1. Technical Field of the Invention
The present invention relates to an optical beam scanning apparatus and an image forming apparatus equipped with the optical beam scanning apparatus, and more particularly, to an optical beam scanning apparatus with a compact optical system to direct a light flux, which is reflected on a deflecting surface of a polygon mirror, to a horizontal synchronization sensor, and an image forming apparatus equipped with the optical beam scanning apparatus.
2. Description of the Related Art
Image forming apparatuses employing an electrophotographic method, such as a laser printer, a digital copying machine, a laser facsimile machine and so on, each have an optical beam scanning apparatus for forming an electrostatic latent image on a photoconductive drum by irradiating and scanning a surface of the photoconductive drum with a laser beam (light beam).
In recent years, a tandem color apparatus has been proposed in addition to a monochrome apparatus equipped with a scanning optical system using a single light source, and in addition, a method for use in the tandem color apparatus has been proposed, which increases the number of laser beams to be scanned one time using a plurality of light sources (laser diodes) arranged in a single laser unit for the purpose of increasing the scan speed on a surface of a photoconductive drum (multi-beam method). In the multi-beam method, a plurality of beams for each of color components (for example, yellow, magenta, cyan and black) emitted from each light source are processed to be combined into a single integrated beam in an pre-deflection optical system, and then the single integrated beam is incident on a polygon mirror. The polygon mirror deflects the incident beam which in turn passes through an fθ lens constituting an post-deflection optical system to be separated into beams for respective color components to be irradiated on respective photoconductive drums corresponding to the respective color components.
In the prior art, for example, in a monochrome apparatus using a single beam, a portion of light flux reflected on a deflection surface of a polygon mirror is used as a horizontal synchronization adjusting signal in the order to align a recording position of a scan optical system (that is, in the order to take a horizontal synchronization). For this reason, a horizontal synchronization sensor for detecting the horizontal synchronization adjusting signal is required to be arranged at a position corresponding to a position of an image plane of a photoconductive drum in the vicinity of the photoconductive drum. However, the arrangement of the horizontal synchronization sensor at such a position may increase the unit size. Accordingly, in the order to provide a compact unit in which the horizontal synchronization sensor is arranged, a horizontal synchronization reflecting mirror is arranged in the middle of an optical path when a portion of light flux reflected on the deflection surface of the polygon mirror is directed to the horizontal synchronization sensor.
In addition, in case of using a multi-beam method, in the order to align a recording position of a scan optical system, an fθ characteristic and a beam diameter on a horizontal synchronization sensor are additionally required to be equal to fθ characteristic and a beam diameter on an image plane of a photoconductive drum, respectively.
In this connection, the following related technique has been known as a technique for directing a portion of light flux reflected on a deflection surface of a polygon mirror to a horizontal synchronization sensor. JP-A-6-3610 discloses a technique for totally reflecting a light flux from a polygon mirror and directing the reflected light flux to a horizontal synchronization sensor to detect an image recording position by means of, for example, a prism integrated with an fθ leans. According to the technique disclosed in JP-A-6-3610, it is possible to provide a compact scan optical system with reduced costs, which deflects a light flux for a horizontal synchronization sensor.
In addition, JP-A-5-134197 discloses a technique in which a synchronization detection lens for imaging a light beam to a horizontal synchronization sensor is formed to be integrated with a portion of a scan lens. According to the technique disclosed in JP-A-5-134197, it is possible to eliminate a dedicated synchronization detection lens for imaging a scanning light beam to a horizontal synchronization sensor for synchronization detection, thereby achieving cost-cutting and reduction of power consumption for attachment operation.
In addition, JP-A-9-203872 discloses a technique in which an optical path bending part, which is an optical scanning lens constituting at least part of a scanning image forming lens and reflects a synchronizing light on an end part of a scanning start side of a deflected light flux in a main scanning corresponding direction, is constituted so as to bend the optical path so that the reflected synchronizing light passes through the lens in the main scanning corresponding direction. According to the technique disclosed in JP-A-9-203872, it is possible to greatly increase the degree of freedom of design of the optical scanning device by making it possible to use a completely new layout for a synchronizing light detecting element of the optical scanning device.
In addition, JP-A-9-243941 discloses a technique in which a mirror part is integrally formed with a portion of a lens for focusing a laser beam to be imaged on a photoconductor, and the laser beam is reflected by the mirror part and is incident on a photodiode. According to the technique disclosed in JP-A-9-243941, it is possible to reduce the number of parts related to an optical system, to reduce costs of parts and assembly costs, and to further improve reliability of an optical device.
In addition, JP-A-9-101471 discloses a technique in which a lens constituting a scanning image forming lens is provided with a reflection part at a part which does not contribute to optical scanning on a scanned surface on the side of the scanned surface at an end part where a deflected light flux is received in a deflecting direction. According to the technique disclosed in JP-A-9-101471, it is possible to compactly arrange an optical system which includes a light receiving element for detecting synchronized light.
However, although the techniques disclosed in JP-A-6-3610 and JP-A-5-134197 may be applied to a single beam, they are difficult to be applied to a multi-beam (in the case of an optical system using a light source having a plurality of emission points) since an fθ characteristic and a beam diameter on a horizontal synchronization sensor are not equal to an fθ characteristic and a beam diameter on an image plane of a photoconductive drum, respectively. In addition, even if the fθ characteristic and the beam diameter on the horizontal synchronization sensor are equal to the fθ characteristic and the beam diameter on the image plane of the photoconductive drum, respectively, the technique disclosed in JP-A-6-3610 cannot avoid increase in the size of a unit since the number of times the light flux reflected on the deflecting surface of the polygon mirror is totally reflected by the prism integrated with the fθ lens or a synchronization detecting lens is one.
The technique disclosed in JP-A-9-203872 cannot decide a position at an incident surface since this technique has the configuration where the light beam is incident on a flange part at the side of the incident surface. In addition, this technique is disadvantageous in manufacturing cost since both end surfaces in a lens longitudinal direction are taken as optical surfaces and accordingly a gate through which resin for molding is flown out is required to be arranged at a different position.
The technique disclosed in JP-A-9-243941 incurs high manufacturing costs of an optical scanning apparatus since a reflecting surface has to be subjected to a vacuum evaporation processing (of metal). The technique disclosed in JP-A-9-101471 has a disadvantage in that a height in a sub-scanning direction in an optical system in which a plurality of light fluxes are passed with different heights in the sub-scanning direction increases although two totally reflecting surfaces are arranged in the sub-scanning direction.